The broad field of elastomeric block copolymers is well known in the art as are pressure-sensitive adhesive formulations containing those block copolymers. Of particular interest are such adhesive compositions which provide high temperature resistance and good tack properties, even after the low radiation doses often used to cure the adhesive compositions. For a number of years efforts were directed to modifications of the block copolymer component which would provide increased processing speed but which would retain desirable adhesive properties.
In U.S. Pat. No. 4,152,231 there are disclosed cured polymer compositions possessing attractive cohesive strength at high temperature which are prepared by radiation curing of a polymer composition in an inert atmosphere. The polymer composition comprised a linear or radical conjugated diene polymer characterized by a number of homopolymer arms, or hydrogenated derivatives thereof. Also present as an essential component was a di- or tetrafunctional acrylate or methacrylate selected from acrylic acid or methacrylic acid esters of polyols, and a tackifying resin could be present as an optional component.
The preferred block copolymers of U.S. Pat. No. 4,152,231 were star polymers with up to 30 arms of substantially linear unsaturated polymer extending from a coupling agent nucleus which was preferably a poly(divinylbenzene) nucleus. The polymers were to have a molecular weight within specified ranges and rather specific amounts of radiation were to be employed for curing.
U.S. Pat. No. 4,163,764 describes other star-shaped block copolymers having arms initially produced from vinylaromatic hydrocarbon followed by formation of a second block of polymerized conjugated alkadiene. This reference does not disclose radiation curable compositions containing the block copolymers or enhancement of sensitivity to radiation curing of the polymers. U.S. Pat. Nos. 4,391,949 and 4,444,953 generally disclose block copolymers and the utility thereof in adhesive compositions. However, these references are not concerned with radiation curable adhesive compositions or enhancement of sensitivity to radiation curing of the block copolymer component.
It is known that block copolymers to be cured by radiation should contain polyisoprene midblocks in order to provide good processability. Most if not all commercially available styrene-based block copolymers contain polyisoprene midblocks if intended for radiation curable compositions.
A number of references are concerned with polystyrene-polyisoprene-polystyrene block copolymers curable by radiation. See, for example, Erickson, Adhesive Age, April 1986, pp. 22-24 and Ewins et al., TAPPI Journal, June 1988, pp. 155-158. These and other references indicate the conception that radiation curable block copolymer components of adhesive formulations should contain polymeric arms prepared from styrene and a branched conjugated alkadiene isoprene. Moreover, the composition should contain at least one additional multifunctional crosslinking agent and a tackifying resin. The choice of tackifying resin was said to be of particular importance with compositions designed to be cured by electron beam (EB) radiation.
In U.S. Pat. No. 5,066,728 there is disclosed a crosslinkable block copolymer having endblocks of polymerized phenylbutadiene and midblocks of polyisoprene or polybutadiene. Crosslinking under the influence of radiation takes place primarily in the endblocks.
Despite the knowledge of the art relative to adhesive compositions containing radiation curable block copolymers, it would yet be of advantage to provide block copolymers of relatively simple structure which demonstrate improved sensitivity for UV or EB radiation in hot melt compositions or solutions. More preferably, said polymers should also show good melt viscosity for use in hot melt, radiation curable adhesive compositions.